In recent years, vehicles have become widespread that employ batteries as sources of propulsion power, these being both electric automobiles that employ batteries as sources for all of their propulsion power and so-called hybrid automobiles that employ batteries as sources for part of their propulsion power. Along with the increasing popularity of vehicles of this type, contactless charging systems are appearing that, when a battery with which such a vehicle is equipped is to be charged up, perform wireless changing in a contactless manner without utilizing any electricity transmission cable physically connected between a charger of a charging facility and the vehicle.
With this type of contactless charging system, a power supply device (hereinafter termed a “contactless charging device”) including a power transmission part (i.e. a power transmission coil) is installed in the ground surface of a parking space or the like, and a power reception device (hereinafter termed an “onboard power reception device”) having a power reception part (i.e. a power reception coil) is disposed under the floor of a vehicle. And it is arranged for the power reception part disposed in the vehicle to oppose the power transmission part installed in the ground surface, so that charging up of the battery is performed. One example of such a device for wirelessly supplying electrical power according to this type of contactless method employs electromagnetic induction.
With such a contactless charging system, if contactless charging is performed in a state with some metallic body present on top of the power transmission part (i.e. the power transmission coil) of the contactless charging device, then this metallic body experiences heating due to the high output electromagnetic waves. Contactless charging in this type of state not only can lead to a fault in the contactless charging device, but can also be a cause of fire. The occurrence of this type of situation is a problem from the point of view of safety during the use of a contactless charging device.
Accordingly, techniques of various kinds have been proposed for detecting that a metallic body (hereinafter this sort of metallic body will simply be termed a “body” or a “foreign body”) is present above the power transmission part (i.e. the power transmission coil) of a contactless charging device. One among these proposed techniques employs the result of detection by a temperature sensor (refer to Patent Document #1, hereinafter termed the “prior art example”). In the technique of this prior art example, in a contactless charging device that employs the electromagnetic induction method, along with a first temperature detection circuit measuring the temperature around the power transmission coil via a thermistor, also a second temperature detection circuit measures the environmental temperature via a thermistor. And, if the value obtained by subtracting the environmental temperature from the temperature around the power transmission coil is greater than a threshold value which is set in advance, then it is determined that a metallic body is present over the power transmission part, and the supply of power is stopped or a notification of anomaly is supplied to the user.